Bimodal Distributions of Two Component Metal Oxide Aerosols

Binary aerosols consisting of ZnO/MgO and CuO/MgO were produced by passing micron-sized metal salt particles, uniform in composition, through a premixed flat flame. The mass distribution as a function of aerodynamic diameter was measured by sampling with an impactor followed by elemental analysis of the material on each stage with inductively coupled plasma-atomic emission spectrometry. The mass distribution of the precursor particles entering the flame was unimodal. The mass distributions of the particles leaving the flame were bimodal for both Zn and Mg in the ZnO/MgO system and for Mg in the CuO/MgO system. The presence of the large mode was attributed to incomplete vaporization of the precursor. The small mode probably resulted from homogeneous nucleation. Transmission electron micrographs of the CuO/MgO particles showed coarse unagglomerated particles in the large mode and agglomerates of very fine particles in the small mode, indicating that two aerosol formation processes, solid phase reaction and gas-to-particle conversion, occur simultaneously. At the temperatures of the flame (T max 1350 K), vapor pressures of the oxides were very low, suggesting that chemical reaction and vaporization of the metal salt precursor compounds are important factors in determining the distribution of individual oxide species as a function of size.